1. Technical Field
The present invention relates to a substrate having an aluminum nitride (AlN) layer and a method for manufacturing an aluminum nitride (AlN) layer in which an aluminum nitride (AlN) layer is annealed in a high temperature mixed gas of nitrogen/carbon monoxide (N2/CO) atmosphere to achieve a higher quality aluminum nitride crystalline layer (hereinafter, “AlN layer”).
2. Background Art
Ultraviolet light-emitting elements are garnering wide attention as next generation light sources as a replacement for fluorescent lights, for use in high density DVDs, for lasers for use in chemistry, for breaking down pollutants by photocatalysis, for use in He—Cd lasers, as a replacement for mercury lamps, and the like. Such ultraviolet light-emitting elements are made of an AlGaN nitride semiconductor known as a wide gap semiconductor, and are layered on a substrate of a different material such as sapphire.
However, sapphire has a large lattice mismatch with AlGaN, which results in a large number of threading dislocations to form a non-light-emitting recombination center, which results in a marked reduction in internal quantum efficiency.
By contrast, AlN has a lattice constant close to that of AlGaN, and is transparent up to the ultraviolet region of 200 nm, and thus, AlN does not absorb emitted ultraviolet rays, allowing ultraviolet light to be efficiently radiated outside. However, monocrystalline AlN crystals are expensive and can only be obtained in 1 inch increments, thereby making it unsuited for use as a substrate material for an ultraviolet light-emitting element. Sapphire can obtained in 6 inch increments at a low cost. Considering such a situation, if it were possible to form a high quality AlN monocrystalline layer on a sapphire substrate, then an AlGaN light-emitting element using this as a substrate could be formed by quasi-homoepitaxial growth, thereby allowing an ultraviolet light-emitting element having a low crystalline defect concentration to be formed at a low cost.
However, AlN has a large lattice mismatch with sapphire, and thus, an AlN layer grown on sapphire has a large number of threading dislocations. Thus, sapphire is unsuitable for use as a substrate for an AlGaN light-emitting element.
To solve this issue, a method is proposed in which an AlN layer is grown on sapphire by MOVPE, and then, the AlN layer is annealed so as to disperse oxygen atoms in the AlN layer to reduce dislocations (see, Patent Document 1, for example). Also, a method in which an AlN layer is grown by MOVPE on a sapphire substrate that has been provided with grooves, and dislocations are mitigated in the process of filling the grooves (see Non-Patent Document 1, for example).